I thought so too, after the troll. I've changed my mind.
Or, did you mean that we knew that it was for practice at keeping things transfigured thirty chapters ago? If so, I just missed it.
I was thinking of this question in chapter 91:
"Is that a usual tactic, by the way?" Harry said, voice still odd. "Carrying something large Transfigured into something small to use as a weapon? Or is that a usual exercise for Transfiguration practice?"
There are a scattering of other discussions of the usefulness of practice, and I thought there was a paragraph that explicitly stated the theory that Dumbledore had given him the rock when he knew that McGonagall was right there, so that Harry would come up with the idea of Transfiguring it, and would thus practice, but I'm not finding it easily.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 114, and also, as a special case due to the exceptionally close posting times, chapter 115.
There is a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)